In such apparatus it is possible for the inlet to the cyclone (or a passage leading from one cyclone to another) to become blocked due to a build up of dust and dirt drawn into the apparatus and not separated by the cyclone action. This reduces the airflow through the cyclone and suction producing fan which becomes cumulative, since reduced airflow reduces the separation efficiency of the cyclone on other dust and dirt entering the cyclone, thereby enabling this material to exit the cyclone with the air stream instead of remaining in the dust collecting region of the apparatus.
Where the airflow is also designed to cool the electric motor driving the suction fan, the reduced airflow through the apparatus could cause the motor to overheat and even burn out.
It is an object of the present invention to provide a means by which an airflow is maintained over the fan motor even if the separation apparatus has become partially or completely blocked upstream of the fan.